1. Field of the Invention
The invention pertains to the field of inspection of manufactured items. More particularly, the invention pertains to a method and apparatus for detection of missing, mislocated or defective chain links or other parts, using a scanning engine.
2. Description of Related Art
Missing link detection is a significant issue in regard to chain assembly of parallel-link type chains. As chain designs are forced toward smaller packages, chain design safety factors are decreasing. Where, at one time, a single missing link could be tolerated for most real-world applications, this is no longer the case with many current production chains. Today, a single missing chain link could easily result in a catastrophic field failure, and the resulting cost and customer relation issues that inevitably follow.
Most, but not all, production chain assembly machines have a missing link detector system installed. However, current production systems have significant shortcomings. Typically, such systems are expensive, difficult to set up, and very time consuming to maintain. More importantly, current systems have proven to be unreliable at times. As such, chains typically receive at least one and sometimes multiple visual inspections before shipment, to assure that no links are missing.
Fundamentally, the prior art missing link systems are quite simple—usually an electromechanical system relying on physical contact with the chain links, as shown in FIG. 1. As chain 1 runs by an inspection station, fingers 3 and 5 contact the backs of the links. If a link 2 is present, then finger 3 closes switch 4 and electrically indicates the presence of a link. If a link 7 is missing, the finger 5 falls into the open area between links and opens switch 6, signaling a missing link.
The sensors are usually located at the final assembly station just prior to pin insertion. When properly aligned and calibrated, the system serves its purpose well. Unfortunately, in a production environment the system is susceptible to many problems—the setup and alignment of the system is quite tedious, and the mechanical parts become clogged with dirt and metal chips, which causes false missing link indications. Most problematic, however, is the inevitable wear that occurs when sensing parts continuously contact the links as they move through the assembly machine. Despite the sensing parts having extremely hard (and expensive) carbide material tips, the hardened chain links eventually wear into the tips. When the wear becomes excessive, the system fails.
Such mechanical systems can, at most, detect links which are truly “missing”—that is, not present at all. They are less capable, or incapable, of detecting mislocated links or links of incorrect type.
One patent on inspection of chains using optical technology, assigned to BorgWarner Corporation, the assignee of the present application, is Ledvina and Mead, U.S. Pat. No. 4,509,323. This patent distinguishes between two types of links in a parallel link chain by reflecting a light off the backs of the links, and reading the reflected light in two sets of sensors, set at different angles. One type of link is made with a flat back, the other with a crowned back. The flat back link reflects light in one direction, toward one set of sensors, the crowned back in many directions, toward both sets of sensors, and thus an “out of place” link can be detected by which sensors are illuminated. Since each group of parallel links is supposed to be all of one type, any mixture of types would mean an “out of place” link, and stop the machine. The method does not use bar-code reader technology, and is not applicable to chains with only one kind of link.
Bar Code Reader Technology
Obviously there is a great need in the art for a truly reliable missing link detection system. The present invention provides a solution to this problem, utilizing standard bar code reader equipment to detect missing chain links.
Bar code technology is currently very widely used for its ability to read black and white labels. Every supermarket and many, if not most, stores use bar codes on products to eliminate manual entry of prices and track inventory. Package delivery services such as UPS and Federal Express use optical bar code readers each including a scanning engine to track packages, and so on. Bar code reader scanning engines are now used for component identification in many industries.
There are two common types of bar code readers. The first is based on mechanical scanning laser beam technology. FIG. 2 shows a schematic view of a laser bar code reader and the operation of this type of reader. A laser 14 is directed at a continuously rotating, multi-sided mirror 16. As the mirror rotates, the geometry of the system projects a continuously scanning laser line onto the target (i.e., the bar code). The reflected light is then detected and interpreted by a reflected light detector system 18. Because laser light is polarized and very intense, the system has great depth-of-field and is insensitive to target variations. Unfortunately, mechanical scanning laser based bar code readers are quite expensive, compared to the alternatives. Also, because they have moving parts, laser based bar code readers are quite fragile and must be isolated from vibration to function properly. More particularly, even under controlled conditions, laser bar code readers typically have a finite useful life.
The second common type of bar code reader uses a different technology that is more suited to harsh environments. These bar code readers are based on Charge Coupled Device (CCD) technology and have no moving parts. A schematic view and the operation of this type of bar code reader is shown in FIG. 3. In the CCD bar code reader, an array of light sensitive diodes 22 is used to view a fixed scan line outside the reader. A lens system 24 is used to focus the incoming light onto the diode array. In this system, typically the light source used to illuminate the target area is a Light Emitting Diode (LED) array 20. The light sensitive diode array 22 is illuminated with an image of the scan line, which is approximately 0.040″ wide. The output of the diode array is continuously scanned, digitized, and interpreted. This method is similar to that used in the common digital camera, except that only a single line is viewed by the bar code reader.